NASA’s ‘ET’ suffered arsenic poisoning!

Published: 7 December 2010(GMT+10)

NASA published a short and cryptic press release on 29 November: they commented
that they were arranging a news conference for 2 December “to discuss an astrobiology
finding that will impact the search for evidence of extraterrestrial life.”1 The obviously intended effect
occurred pretty much immediately: the media and internet were set buzzing with widespread
speculation as to what exactly this could mean, ranging from the prosaic to the
preposterous.

It turns out it was much more prosaic than even many more circumspect commentators
had speculated: they discovered evidence that a bacterial strain found on Earth
was capable of metabolizing arsenic (As) and incorporating it into structural molecules
of the cell.2 Researchers
found a strain of gamma proteobacteria known as GFAJ-1, found at Mono Lake in California,
can feed on As and incorporate it into its biological structures when there was
a shortage of phosphorus (P).3

Knowledge that As is positively utilized in biology has only come to light in the
last 15 years. Arsenic has been shown to be an important part in both respiration
and photosynthesis for a wide variety of bacteria,4
including gammaproteobacteria.5
However, this latest finding is qualitatively different because rather than merely
using As in its inorganic forms (arsenite (AsO3–) and
arsenate (AsO43–)) to provide energy, GFAJ-1 incorporates
arsenate into biochemical structures normally occupied by phosphate (PO43–).

This is a big deal because P is not a trace element—it is essential for countless
processes that are ubiquitous in biology. Some of the most recognizable are: providing
structure for DNA and
RNA, and as an essential component of the universal energy “currency”
in biology, ATP (adenosine triphosphate). Life cannot
survive without those three molecules, and that’s the tip of the iceberg of
the essential structures and processes P is needed for to survive. Life as we understood
it before this announcement could not survive without P, but we have found evidence
that some life may be able to.

Why arsenic?

It may seem counter-intuitive at first that As could substitute for P because it’s
popularly known as a poison. However, toxicity is not a property inherent to any
substance; rather it’s always a matter of concentration.6 There is a concentration below which As is not toxic.

One reason As is so effective as a poison is it readily substitutes for P in many
molecules essential for biological function, thereby disrupting many biological
processes. It readily substitutes for P because it has similar chemical properties
to P, which can be inferred simply from a periodic table (both are Main Group 5).
Any elements in the periodic table that line up vertically have many similar properties,
and are often capable of many of the same types of reactions. But given the fine-tuning—down
to single-molecule chemistry—of
many biochemical processes, this is enough to block vital reaction pathways.

These bacteria had a 38% drop in growth rate when fed on arsenic rather than phosphorus,
and were structurally weaker.

For another example of how chemically very similar elements or compounds can have
vastly different biochemistries due to fine-tuning, see this explanation of
potassium ion pumps, which block the smaller but otherwise very similar
sodium ion. The same applies with very similar amino acids that have ordinary hydrocarbon
side chains—in living creatures, this difference can be crucial, so there
are ingenious double-sieve
enzymes to separate them.

There is something similar in Group 2: magnesium and especially calcium are vital
for life; strontium (below calcium) is tolerable except the radioactive isotopes,
while barium (below strontium) and beryllium (above magnesium) are toxic.

It’s a similar reason to why many people speculate about ‘silicon-based
life forms’ as a possible alternative organic base to carbon (C)—silicon
(Si) is directly below carbon on the periodic table. But silicon is not
a good biological analogue for carbon because silicon is a much larger atom, which
means that Si-to-Si covalent bonds are much less stable than C-to-C bonds, and Si
bonds less well to many other types of atoms. For example, while hydrocarbons and
water don’t mix (oil and water), silanes, hydrogen-silicon compounds, react
readily with water. Another problem is that while CO2 is a water-soluble
gas, SiO2 (silica) is an insoluble solid (e.g. quartz).

Arsenic is a much better analogue for P, but As compounds are not as stable for
metabolism as P compounds,7
which is necessary in biology because of the extreme fine-tuning of life and its
single-molecule chemistry.8

Why the ET buzz?

Since this discovery provides us with an alternative biochemistry to what we are
used to, it supposedly provides more ways that life could arise from non-life both
here and elsewhere in the universe. However, GFAJ-1 has not completely done away
with P biochemistry. Paul Davies agrees:

“This organism has dual capability. It can grow with either phosphorous or
arsenic. That makes it very peculiar, though it falls short of being some form of
truly ‘alien’ life belonging to a different tree of life with a separate
origin. However, GFAJ-1 may be a pointer to even weirder organisms. The holy grail
would be a microbe that contained no phosphorus at all.”9

This find hasn’t even served to uncover a completely different biology to
what we know on earth, much less provide evidence for the existence of extraterrestrial
life.

However, the original research goes a bit further. The researchers found that GFAJ-1
had a 38% drop in growth rate when fed on As rather than P.10 Moreover, the bacteria fed on As appeared structurally
weaker than those fed on P, demonstrated by the robust integrity of the P-fed cells
in comparison to the As-fed cells when prepared for analysis.10 Thus
GFAJ-1 still grows much faster and the organisms are healthier when fed P rather
than As. Therefore GFAJ-1’s utilization of As compounds appears to be a sub-optimal
substitute to P biochemistry in GFAJ-1—when P is lacking it can utilize As
to grow, but P remains the preferred option when it is available. This is similar
in principle to
Lenski’s citrate-eating bacteria, which turn out to have a disabled
off-switch so that an already-existing citrate-digesting pathway is left on even
under aerobic conditions.

Moreover, gamma proteobacteria are a well-known form of earth-based bacteria
with some species already known to use As in their biology, so GFAJ-1 is not as
isolated from the rest of biology as one might be led to think by the news reports.
As big a discovery as this is it still doesn’t break the unity of the biosphere;
it merely extends our knowledge of what it can do.

In essence, the “astrobiologists” were merely announcing an earthly
experiment in artificial selection (echoing
natural selection—which creationists not only accept, but also
described before Darwin did). From the researchers’ report,3
we see they moved an existing microbe from a Californian lake to their
lab, then gradually reduced the available phosphate, while feeding the culture arsenate.
Some microbes in the population had a pre-existing ability to tolerate
As—they were the only microbes to survive. If the As-tolerance mechanism was
the result of a mutation, it would likely come at a cost. Therefore if these poison-tolerant
microbes were returned to the wild, they would be at a competitive disadvantage
compared to their wild cousins—see
Evolution in a Petri dish?

Conclusions

This whole episode seems like it was a well-orchestrated publicity stunt by NASA
to get some attention for itself. While this find is a very important discovery
about biology, it’s an important discovery about earth biology at
its functional limits. Most biologists won’t abandon a single common
ancestor for life based on this find because this is a modification of known
biology that operates at suboptimal levels. A better analogy would be so-called “superbugs”,
which are deadly in a hospital environment full of antibiotics, but can’t
compete with their respective wild types outside of the hospital. This find hasn’t
even served to uncover a completely different biology to what we know on earth,
much less provide evidence for the existence of extraterrestrial life.

Postscript 1 (10 December 2010): In information just to hand (see
http://scienceblogs.com/webeasties/2010/12/guest_post_arsenate-based_dna.php
and
http://rrresearch.blogspot.com/2010/12/arsenic-associated-bacteria-nasas.html),
two Ph.D. scientists have raised serious doubts about the claim that the bacteria
in question have an arsenic (rather than phosphate) backbone to their DNA. One of
them calls it a ‘big idea with big holes’. The media have widely promoted
the claim, which would have reinforced ET ideas in the minds of millions—will
they in due course widely publish on these growing doubts? If history is any guide,
it’s very unlikely. Notions of ‘alien organisms’ sell papers,
whereas being reminded of the fallibility of science (even when dealing with observations
on the present-day world) is seen as boring in contrast.—Ed.

Postscript 2 (23 December 2010): Science by media—NASA can give it but can’t take it?

As we remarked earlier, the response from scientific skeptics of the arsenic-eating
bacteria “find” was fast and hard-hitting. They pointed out that the
methodology was flawed, the predicted chemistry was suspect, and even that there
is no reason for GFAJ-1 to evolve the ability to eat arsenic since its environment
is one of the most phosphate-rich in the world (see
Alex Bradley s and
Rosie Redfield s responses). The Lead author,
Felisa Wolfe-Simon, has issued a response to the criticisms on her
blog, which has in turn garnered further responses from Rosie Redfield (see
here and
here).

However, NASA employed its usual tactic of silence and “letting it sort itself
out in the scientific literature” to play down the criticism:

“When NASA spokesman Dwayne Brown was asked about public criticisms of the
paper in the blogosphere, he noted that the article was peer-reviewed and published
in one of the most prestigious scientific journals. He added that Wolfe-Simon will
not be responding to individual criticisms, as the agency doesn’t feel it
is appropriate to debate the science using the media and bloggers. Instead, it believes
that should be done in scientific publications.”11

NASA splashes this all over the media, deliberately creating a
massive media buzz before the official press conference, and then has the gall to
suggest this? If NASA wanted this to be played out in the peer-reviewed
literature, they should’ve left it there. Or at the very least, they should
not have released a
cryptic press statement suggesting this finding “will impact the search
for evidence of extraterrestrial life.” But instead, Brown
blames the bloggers for misrepresenting the press release:

“The real issue is that the reporting world has changed because of the Internet/bloggers/social
media, etc. A ‘buzz’ term like ET will have anyone with a computer put
out anything they want or feel. NASA DID NOT HYPE anything—others did.
Credible media organizations have not questioned NASA about any text. Bloggers and
social media have … it’s what makes our country great—FREEDOM
OF SPEECH [emphasis added].”12

Can anyone seriously believe NASA didn’t intend for this
to be hyped in the media? Yes, “ET” is a buzz term; but they knew that.
Moreover, those “bloggers” include Redfield (she runs a microbiology
lab at the University of British Columbia), Bradley (A Harvard biogeochemist—this
finding is exactly his research specialty). NASA lapped up the
media coverage when it was positive, but once qualified scientists
such as Redfield and Bradley, and a
dozen others interviewed by science journalist
Carl Zimmer (who also gave the
NASA researchers opportunity to comment, but they refused to), came
out almost unanimously criticizing the findings, they claim the scientists should
reserve their comments for the peer-reviewed literature!

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Readers’ comments

Graham P.,New Zealand, 7 December 2010

Excellent piece.

Douglas A.,United States, 7 December 2010

Where does the quote "These bacteria had a 38% drop in growth rate when fed on arsenic rather than phosphorus, and were structurally weaker." come from (no citation…

Shaun Doyle responds

Thank you for your comments. I didn’t directly quote the study on those points because I merely referenced it. I got the growth rate reduction from comparing the recorded average maximum growth rates of the P-fed bacteria with the As-fed bacteria. For the P-fed bacteria it was 0.86 day-1 and for As-fed bacteria it was 0.53 day-1. For a direct quote, see p. 2 of Wolfe-Simon et al.:

“GFAJ-1 grew at an average μmax of 0.53 day-1 under +As/−P, increasing by over 20-fold in cell numbers after six days. It also grew faster and more extensively with the addition of 1.5 mM PO43- (−As/+P, μmaxof 0.86 day-1, Fig. 1A, B) [emphases added].”

“There was variation in the total As content of the +As/−P cells, possibly a result of collection during stationary phase and losses during the repeated centrifugations and washing cycles due to the potential instability of the cellular structures given their swollen state (Fig. 2C, E). In contrast, the integrity of the −As/+P cells appeared robust (Fig. 2D) and thus intracellular P measured for these cells likely reflects their content [emphasis added].”

I hope this helps,

Shaun Doyle.

Jeremy W.,Australia, 9 December 2010

This ‘discovery’ sounds more like NASA is wanting to make sure funding for ‘Astrobiology’ is kept or increased.

Jack B.,Australia, 9 December 2010

It seems that the word ‘Astrobiology’ has just appeared on the table—Study of life in outer space. It makes it sound like there already is life in outer space. How can they have a name for something they haven't directly observed yet?